The present invention relates to an apparatus and to a method for determining hematocrit, and more particularly to coincidence correction in such an apparatus and method.
Hematocrit is the percentage of packed red blood cells in a sample of whole blood. Hematocrit measurement is commonly performed within a blood cell counting instrument. Examples of hematocrit measurement apparatus and blood cell counting apparatus are found, for example, respectively in U.S. Pat. No. 4,068,169 issued Jan. 10, 1978 to Henry R. Angel and Bernard O. Bachenheimer and U.S. Pat. No. 3,921,066 issued Nov. 18, 1975 to Henry R. Angel and James W. Hennessy. Both patents have been assigned to the assignee herein, and their disclosures are incorporated herein by reference. Different forms of hematocrit measurement circuits in a hematology counter are well-known in the art. The above cited patent to Angel and Bachenheimer as well as U.S. Pat. No. 3,828,260 issued to Raymond D. Underwood on Aug. 6, 1974 disclose circuits in which the size of each pulse representing a red blood cell count is measured and signals each indicative of volume of a blood cell volume are totalized. This is referred to as direct measurement of hematocrit. Another form of hematocrit measurement apparatus is disclosed in U.S. Pat. No. 3,439,267 issued Aug. 15, 1969 to Wallace H. Coulter and Walter R. Hogg. Hematocrit is calculated as a function of mean corpuscular volume times the red blood cell count. This type of circuit is distinguished from the type in which cell volumes are totalized.
The red blood cell count has a coincidence correction factor applied thereto. Red blood cell count pulses are produced by impedance changes in an aperture resulting from blood cells passing therethrough. More than one cell may coincidentially pass through the aperture at one time, so a coincidence correction factor must be applied to a "raw" red blood cell count to provide an actual count. Many coincidence correction circuits are well-known in the art for use in producing red blood cell counts. It has long been known that applying a coincidence correction factor which is the function of the magnitude of a red blood cell count is desirable. However, coincidence correction has heretofore not been applied to direct measurement of hematocrit.
When a red blood cell is not counted due to coincidence, its size is not measured, and a pulse indicative thereof is not provided for incrementing accumulation means. The effect of coincidence error may not be highly significant when performing counts on samples in which the sample is blood highly diluted in saline diluent (e.g. 1:150,000) or in which the red blood cell count is low. Significance of the effect increases with density of the blood dilution or with level of red blood cell count. It has been discovered that coincidence correction has utility in direct measurement of hematocrit.